This invention relates to a novel method for the application of a thin layer of a hardenable liquid to the outer surface of a cylinder. While not limited thereto, the method has special applicability for the application of a protective overcoating material to electrostatographic photo-receptors. The art of electrostatographic copying, originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves, as an initial step, the uniform charging of a plate or drum comprised of a conductive substrate normally bearing on its surface a non-conductive barrier layer which is covered by a layer of photoconductive insulating material. The charged plate is exposed to activating radiation in imagewise configuration which results in dissipation of the electrostatic charge in the exposed areas while the non-exposed areas retain the charge in a pattern known as the latent image. The latent image is developed by contacting it with an electroscopic marking material commonly referred to as toner. This material is electrically attracted to the latent image which is, by definition, in the configuration of those portions of the photoreceptor which were not exposed to the activating radiation. The toner image may be subsequently transferred to paper and fused to it to form a permanent copy. Following this, the latent image is erased by discharging the drum and excess toner is cleaned from it to prepare it for the next cycle.
The photoconductive insulating material is characterized in that it has a comparatively high electrical resistance in the dark which resistance decreases significantly upon exposure to activating radation. Both organic materials, such as 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazole), and inorganic materials, such as amorphous selenium, have been successfully used as the photoconductive material in electrostatographic copiers.
In some instances, it has been found to be desirable to overcoat the layer of photoconductive insulating material with a thin layer of an organic material. These overcoatings are applied for a variety of reasons. One reason for using an overcoating is to protect the photoconductive material from physical abrasion during the copying cycle. Overcoatings of this type are disclosed in U.S. Pat. No. 2,860,040 (where polyvinylacetal or polyvinylformal are used) and U.S. Pat. No. 2,886,434 (where inorganic materials such as ZnS, SiO.sub.2 and TiO.sub.2 are used). In certain instances, an overcoating material is applied to the layer of photoconductive material to enhance its cleanability. An example of this type of overcoating is to be found in U.S. Pat. No. 3,793,018 wherein it is disclosed that a hydrophobic bivalent or trivalent metal salt of a half ester of a branched chain or straight chain aliphatic dicarboxylic acid of a mono- or di-ester of a phosphorous oxyacid can be applied to the photoreceptor. Still another use of an overcoating is to protect the photoconductive material from chemical constituents in the ambient which may detrimentally react with it. Copending application Ser. No. 558,027 discloses this type of overcoating. In addition, certain polymeric overcoatings have been found to increase the electrophotographic speed of the photoroceptor. Many of these overcoating materials are applied from their liquid solutions or dispersions. Application of the solution or dispersion with subsequent evaporation of the liquid leaves a thin layer of the overcoating material on the outer surface of the cylinder. When an organic polymeric material is used as the overcoating, it is typically applied from a synthetic latex, i.e. a finely divided emulsion of polymer particles in an aqueous carrier. The carrier normally contains a basic material such as ammonia or an amine, e.g. isopropylamine, to form a salt with weakly ionized carboxylic acid groups on the polymer backbone and thereby increase the solubility of the polymer. Evaporation of the water leaves a thin layer of the polymer.
In order to get a polymer layer of uniform thickness it is, of course, necessary to apply a layer of the dispersion uniformly. One method of applying the liquid to the cylinder involves applying a wetted sponge to the cylinder's surface and rotating the cylinder about its axis of rotation while moving the wetted sponge laterally along the cylinder. This method can be effective in applying a uniform coating but requires a fairly high level of operator skill, especially when the liquid is of the type which hardens rapidly. Another method is to apply the hardenable liquid to the cylinder with a spray gun and allowing it to drain and dry. This method is quite wasteful of material, however, since enough liquid must be applied so all the bubbles generated by the spray drain off.
It would be deisrable, and it is an object of the present invention, to provide a novel process for the application of a thin, uniform layer of a hardenable liquid to the surface of cylindrical substrate.
A further object is to provide such a process which is relatively easy to carry out and employs simple, inexpensive equipment.
An additional object is to provide such a method which is economical in terms of coating liquid consumption.